Split vector systems for ultra-targeted gene delivery: a contrivance to achieve ethical assurance of somatic gene therapy in vivo.

Tightly controlled spatial localisation of therapeutic gene delivery is essential to maximize the benefits of somatic gene therapy in vivo and to reduce its undesired effects on the 'bystander' cell populations, most importantly germline cells. Indeed, complete ethical assurance of somatic gene therapy can only be achieved with ultra-targeted gene delivery, which excludes the risk of inadvertent germline gene transfer. Thus, it is desired to supplement existing strategies of physical focusing and biological (cell-specific) targeting of gene delivery with an additional principle for the rigid control over spread of gene transfer within the body. In this paper I advance the concept of 'combinatorial' targeting of therapeutic gene transfer in vivo. I hypothesize that it is possible to engineer complex gene delivery vector systems consisting of several components, each one of them capable of independent spread within the human body but incapable of independent facilitation of gene transfer. As the gene delivery augmented by such split vector systems would be reliant on the simultaneous availability of all the vector system components at a predetermined body site, it is envisaged that higher order reaction kinetics required for the assembly of the functional gene transfer configuration would sharpen spatial localisation of gene transfer via curtailing the blurring effect of the vector spread within the body. A particular implementation of such split vector system could be obtained through supplementing a viral therapeutic gene vector with a separate auxiliary vector carrying a non-integrative and non-replicative form of a gene (e.g., mRNA) coding for a cellular receptor of the therapeutic vector component. Gene-transfer-enabling components of the vector system, which would be delivered separately from the vector component loaded with the therapeutic gene cargo, could also be cell-membrane-insertion-proficient receptors, elements of artificial transmembrane channels capable of nucleic acid transfer or, perhaps, factors modifying existing cellular transmembrane channels (e.g., gap-junctional hemichannels) to serve as conduits for gene entry. In general, there are four possibilities for gene transfer in vivo using a split vector system: (1) simultaneous delivery of a mixture of the vector components to the same body site; (2) sequential delivery of the vector components to the same body site; (3) simultaneous delivery of the vector components to separate body sites; (4) sequential delivery of the vector components to separate body sites. It is hoped that, once experimentally confirmed, the combinatorial principle for tight control over localisation of gene transfer could be the critical element in attaining complete assurance of gene non-delivery to germline cells in somatic gene therapy in vivo.